The invention relates to a cooling apparatus and method that uses carbon dioxide (CO2) to cool, refrigerate or freeze; and more particularly to a portable device preferably embodied in a selfcontained refrigeration system that cools indirectly with liquid carbon dioxide, utilizing the sublimation of CO2 xe2x80x9cdry-snowxe2x80x9d to self-regulate the flow of the CO2 refrigerant in the system.
Carbon dioxide (CO2), in its liquid and solid forms is employed in many commercial, industrial and to some extent residential refrigeration, freezing and cooling applications. Under normal, atmospheric pressure, CO2 can only exist as a gas at ambient temperatures or a chilled solid that sublimates into the gas at minus 109.3 degrees, Fahrenheit. However, if pressurized, gaseous CO2 forms a liquid with a water-like viscosity. CO2 is a desirable refrigerant because it is colorless and typically odorless. It is also chemically inert under ordinary conditions.
Because CO2 is inexpensive to manufacture and has little potential for adverse environmental impact in small quantities, it is often employed as a disposable refrigerant. Several U.S. patent references utilize the unique properties of CO2 in re-chargeable types of refrigeration, freezing and cooling systems.
U.S. Pat. No. 3,561,226 Rubin describes a refrigeration system for vehicles that includes a minimum of moving parts. Rubin ""226 employs a liquid reservoir of carbon dioxide to fill a set of containers with densely packed xe2x80x9csnow.xe2x80x9d The filling of the container with carbon dioxide is activated by a timed solenoid valve, or alternatively a manual valve. CO2 vapor passes out of each container, while CO2 snow is trapped by a screen, backing up the snow into the container. A portable refrigeration system is needed that better utilizes liquid CO2 to more evenly generate a self regulated refrigeration effect, rather than the expansion of liquid CO2 into a trough or bin container.
Similarly, to Rubin ""226, U.S. Pat. No. 4,704,876 to Hill shows a cryogenic refrigeration system that uses carbon dioxide snow formed by spraying CO2 through a series of nozzles within a snow collection compartment. The compartment is open to a refrigerated space, below. This feature is undesirable because the cargo is directly exposed to the frigid CO2 snow, which can result in damage to the cargo, especially if it is perishable fruits or vegetables. Control is achieved with temperature sensors in the refrigerated compartment that opens a valve to deliver more liquid CO2 to the nozzles, or alternatively by manual valving. Hill ""876 also requires that the nozzles remain xe2x80x9cfree and openxe2x80x9d of solid CO2.
U.S. Pat. No. 4,381,649 to Franklin discloses a CO2 snow producer that includes spray nozzles for directing jets of liquid carbon dioxide onto a surface of a heat exchanger. The pressure at the nozzles of Franklin ""649 is approximately 75 psi and the temperature in the supply line to the nozzle is maintained near minus 69xc2x0 F., as a goal. From the Franklin disclosure, control of the system is achieved through what appears to be a manual control valve at the tank of liquid CO2.
Several problems are encountered in Ruben ""226, Franklin ""649 and Hill ""876 when the temperature feedback is disabled or malfunctions, the timer is set incorrectly, or the operator fails to activate or deactivate the system as needed. Direct, on/off control by an operator is especially unreliable in that the system can over-cool, with an over production of CO2 snow risking damage to the refrigerated contents or to the operators. Under cooling can also cause undesirable effects in the unwanted thawing or spoiling of refrigerated articles. A CO2 refrigeration system is needed that employs a simple design but eliminates the need for an on/off CO2 control valve that directly meters the release of CO2 into a refrigerated space.